1. Field of the Invention
This invention relates to a plastic gear molding system and method, in particular relates to a plastic gear molding system and method capable of preventing from partial shrinkage during its molding process.
2. Discussion of the Background
It is well known in a transmission apparatus to have at least one plastic gear that transmits a rotation force of a driving motor to a driven member. For example, in an image forming apparatus such as a copier, a printer, a facsimile and a multi-functioned machine having a plurality of functions, a rotation force of a driving motor is generally transmitted to an image carrier through a driven member contacting the surface of the image carrier or the like for forming a toner image on the surface of the image carrier during its rotation. The conventional plastic gear may also be used in a duplicator, a camera, a video deck and a compact disk player and so on to transmit a rotational force to a driven member thereof.
In recent years, such plastic gears have tended to be rotated at relatively high speeds and so have been subjected to higher external forces. Since the conventional plastic gear is simply constituted by a hub, a gear ring and a web whose ends connect the hub and the gear ring, it has been difficult to meet the necessary level of rigidity and strength required for the plastic gear. It is of course possible to increase both the rigidity and the strength to meet the prescribed level if both a thickness and a size of the background gear are increased.
However, this is costly and a transmission apparatus unavoidably becomes bulky. To increase the rigidity, a plurality of ribs may be symmetrically integrally mounted on both front and rear surfaces of the web in a manner such that each one edge connects with the hub and each another edge connects to the gear ring. However, a diameter of such a plastic gear generally varies during its molding process due to the so called a shrink phenomenon of the plastic. As a result, a peripheral speed of the plastic gear periodically changes when it rotates, and accordingly unevenness of the rotational speed of the driven member may arise.
The present inventors have determined that the shrink phenomenon occurs for the reason hereinbelow explained in detail referring to FIGS. 10 through 12. A background plastic gear 14 includes a cylindrical hub 25 disposed as a core portion thereof and is supported by the shaft 15 illustrated in FIG. 2. The plastic gear 14 further includes a gear ring 27 having substantially concentric with the hub 24 and having a larger diameter than the hub 24, which is disposed at of the hub 25.
The gear ring 27 includes a plurality of gear teeth 26 on an outer circumferential surface thereof. The plastic gear 14 further includes a web 28 constituted by a circular plate whose ends integrally connect the hub 24 and the gear ring 27. A plurality of ribs 28A and 28B are each integrally formed respectively on front and rear sides of the web 28. Each of the plurality of groups of the ribs extends in a radial state from the hub 24 to the gear ring 27.
The ribs 29A formed on a front surface of the web 28 are arranged at a prescribed angular interval around the hub 25. The ribs 29B formed on a rear surface of the web 28 are arranged in a same way as the ribs 29A. Each of the ribs 29A and 29B is symmetrically disposed at both the front and rear surfaces of the web 28. As a result, a perpendicular cross section of the web 28 intersects the cross sections of both the ribs 29A and 29B as illustrated in FIG. 7. Since a partial shrinkage phenomenon occurs at each of portions of the plastic gear 14 where the ribs 28A and 29B are disposed at same angular positions on the front and rear side surfaces of the web 28, during a cooling process of molding, diameters of these portions decrease to be less than that of other portions.
When producing a gear made of a metal by cutting a metal material, such a partial shrinkage phenomenon, of course, does not occur. Such a partial shrinkage phenomenon may occur only in a case that a pair of ribs 28A and 29B are disposed at same angular positions on the front and rear side surfaces of the web 28. The eccentricity of a gear periphery of the conventional plastic gear that includes a pair of six pairs of ribs 29A and 29B respectively formed on the front and rear side surfaces 28A and 28B of the web 28 is illustrated in FIG. 14. As there shown, the diameter of the gear edge circle remarkably changes six times corresponding to the number of the ribs. As a result, the rotational speed of the plastic gear varies six times; thereby unevenness of the rotation speed arises when the conventional plastic gear rotates.
A possible cause of the change in rotational speed of the plastic gear is explained below. A portion of the gear ring 27 and gear teeth 26A, 26B and 26C each mounted on the circumference of the gear ring 27 are typically illustrated in FIG. 15. As there shown, ends of the ribs 29A and 29B are connected to the same portion (shown enlarged for ease of illustration) of the gear ring 27 between the tooth 26B and 26C. A space between the teeth 26B and 26C is illustrated larger than actual for an explaining purpose.
The portion of the gear ring 27 between the teeth 26B and 26C is more indented toward a rotational center of the plastic gear than other portions thereof, since the partial shrink occurs when the plastic gear is molded. Thus, the tooth 29A positioning at a left side of the ribs 29A and 29B inclines on the right and the tooth 29B positioning at a right side of the ribs 29A and 29B inclines to the left as illustrated in FIG. 15.
A gear 26D meshes with the plastic gear 14 as illustrated in FIG. 15. If a pressure angle at a gear connecting portion at which a gear tooth 26D of another gear meshes with the gear tooth 26A is α0, a pressure angle α1, of the gear tooth 26B inclining to the right is larger than α0, A pressure angle α2 of the gear tooth 26C inclining on the left is smaller than α0.
If angular velocities are ω0, ω1, and ω2 correspond to gear portions having the angles of α0, α1, and α2.
The larger the pressure angle, the smaller the angular velocity and the smaller the pressure angle, the larger the angular velocity. Thus, the following relation is established around the ribs 29A and 29B.ω1<ω0<ω2
Thus, when ribs 29A and 29B extend in a radial state, for example, from the rotational center of the gear and are each disposed in a same angular interval, a rotational speed of the gear periodically varies when the plastic gear rotates.
Further, a rotational speed of the conventional driving motor 10 generally varies once per one revolution thereof. Thus, a rotational velocity of the PC drum 1 remarkably changes at a prescribed timing, if a frequency of a change in rotational speed of the conventional driving motor 10 is substantially coincident with that of the plastic gear 14. This is because, cylindrical peaks due to the change in rotational speed of the driving motor 10 and that due to the plastic gear 14 coincide with each other. As a result, unevenness of a toner image (so called the jitter) arises on the surface of the PC drum 1, and the image quality is inferior.
For example, if the driving motor 10 rotates at 1,800 rpm, a frequency of a change in rotational speed is 30 Hz (obtained by dividing 1,800 rpm by 60 seconds). If the number of teeth of the output gear 13 of the driving motor 10 is ten, a number of teeth of a plastic gear 14 that meshes with the output gear is seventy, and a number of ribs 28A and 28B mounted on each of the surfaces of the web 28 of the plastic gear 14 is seven, a frequency of a change in rotational speed of the gear 14 becomes 30 Hz, as is obtained by the following formula.1800 rpm×( 10/70)×( 1/60 sec)×7=30 Hz
Thus, if the peaks of the above-mentioned cycles accord with each other, the change in rotational speed of the PC drum 1 becomes remarkably large at a prescribed timing, since the change in rotational speed of the driving motor is added to that of the plastic gear 14. Thus, the above-mentioned plastic gear may not be used for the transmission device.
Further, a plastic gear may be produced using an injection molding method. In such a method, if a new plastic gear that has a larger or smaller number of ribs than the plastic gear previously used is to be molded using the same mold, the mold is required to be remodeled to produce a different number of the ribs. However, it generally is expensive to remodel the mold that has produced the previous model of the plastic gear, for example, by changing a plastic injection gate, through which molten plastic is poured.
Further, after a plastic gear is molded, it is ejected from the mold generally using a prescribed number of ejecting pins by the following manner.
As illustrated in FIG. 19, the prescribed number of ejecting pins is inserted into guiding holes provided in a core respectively. When the molding is completed, the gate and cavity are separated from the core as illustrated in FIG. 20, and the prescribed number of ejecting pins then automatically push the plastic gear out to eject the plastic gear from the core.
During ejection of the plastic gear, a great amount of resistance is generally generated due to thrusting conflict between a surface of the plastic gear and a gear-cutting portion of the core as noted from FIG. 21. To overcome the great amount of resistance, a prescribed amount of pressure generally is applied to the plastic gear via the plurality of ejecting pins. Thus, the plurality of pins generally have relatively large diameters respectively to cooperatively apply the prescribed amount of pressure. In such a case, each of the plurality of ejecting pins is preferably arranged to contact the plastic gear with it leading end as being as close to the gear bottom as possible, as illustrated in FIG. 22, in order to minimize shearing force to be generated between the pressure and the resistance in the plastic gear as much as possible. To this end, a prescribed plurality of portions of the section of the gear bottom generally are molded to be fat, as illustrated in FIGS. 23A and 23B, and the plurality of ejecting pins contacts the fat sections directly or via a prescribed base plate.
However, such fat section might effect partial shrinks in gear teeth locating at the back side of the fat portion after the molding, thereby resulting in unevenness of radius diameter and rotational speed. This causes a problem such as jitter, for example, in a half tone image when such a plastic gear is utilized as a part of transmission for transmitting a driving force to an image carrier on which an image is formed.